John Bell

Bell's theorem, published in 1964, showed that certain predictions of quantum mechanics cannot be reproduced by local hidden-variable theories. It turned a philosophical debate about quantum reality into an experimentally testable question.

Bell built on the 1935 Einstein-Podolsky-Rosen argument about whether quantum mechanics was complete. His theorem gave a way to test whether local hidden variables could explain quantum correlations.

Bell inequalities are mathematical limits that local hidden-variable theories must obey. Experiments testing entangled particles can violate those limits, matching quantum mechanics rather than local realism.

No, Bell also worked in particle physics and accelerator physics, including at CERN. His fame, though, rests mainly on the 1964 theorem that changed the foundations of quantum mechanics.